1. Field of the Invention
This invention relates generally to a photographic exposure control system having a preset digital light integrator and, more particularly, to a photographic exposure control system having a digital light integrator which is preset prior to exposure to assure an accurate first light detect at the commencement of the exposure interval.
2. Description of the Prior Art
In the photographic art, exposure control systems embodying scanning shutter blade elements which operate to vary exposure aperture areas with time during the exposure interval are well known as indicated by U.S. Pat. No. 3,942,183, entitled "Camera with Pivoting Blades", by George D. Whiteside, issued Mar. 2, 1976, in common assignment herewith. Such scanning shutter blade mechanisms generally include a pair of counter-reciprocating shutter blade elements each having a primary aperture that traverses the optical axis of the camera during the exposure interval. The primary apertures are shaped so that upon overlying one another during countermovement of the blades, there is defined an effective exposure aperture value which increases to a maximum value in a determinate period of time.
Exposure control is generally provided by a pair of secondary photocell apertures in respective shutter blade elements which admit scene light to a photoresponsive element in correspondence with the scene light admitted to the focal plane during shutter blade movement through an exposure cycle. The output from the photoresponsive element is directed to an integrator circuit which triggers upon reaching an integration level corresponding to a desired exposure value to terminate the exposure interval by returning the shutter blade elements back to their initial scene light blocking position.
Shutter mechanisms of the aforementioned type are often arranged to operate in synchronism with a source of artificial illumination such as an electronic flash or strobe. The shutter mechanism and strobe are generally arranged to operate in either a high ambient scene light intensity mode of operation where no artificial illumination is provided or a low ambient scene light intensity mode of operation where the strobe is fired to provide a source of artificial illumination. Under conditions of low ambient scene light intensity where the strobe is expected to be fired, the scanning shutter blade mechanism may be stopped at an aperture value corresponding to the camera-to-subject distance as determined by focusing the objective lens. Systems of the aforementioned type are generally referred to as "follow focus" systems and the maximum aperture to which the scanning shutter blade elements are allowed to open is controlled by the rangefinding or focusing system of the camera in a manner as is more fully disclosed in U.S. Pat. No. 3,977,014, entitled "Follow Focus Exposure Control System with Improved Uniform Trim Control", by Philip R. Norris, issued Aug. 24, 1976, in common assignment herewith.
Such a system need not actually stop the opening shutter blade movement at a maximum effective exposure aperture as determined by the rangefinding or focusing system of the camera, but may instead vary the time at which the strobe light is fired during the opening shutter blade movement as determined in correspondence with the rangefinding or focusing system of the camera. Since the duration of the strobe light is short in comparison with the time required for the shutter blade mechanism to scan from its scene light blocking position to its maximum exposure aperture defining position, the aperture defined by the shutter blade elements at the instant of strobe firing for practical purposes constitutes the effective aperture by which the exposure occurs. Such a system is more fully described in U.S. Pat. No. 3,478,660, entitled "Photographic Apparatus with Flash Exposure Control System", by Edwin H. Land, issued Nov. 18, 1969, in common assignment herewith.
Range responsive flash fire systems of the aforementioned type can also be utilized in conjunction with sonar ranging devices as is more fully described in U.S. Pat. No. 4,188,103, entitled "Range Synchronized Flash Photographic Apparatus and Method for Achieving Optimum Flash Exposure", by Conrad H. Biber et al., issued Feb. 12, 1980, in common assignment herewith. In the aforementioned system, means must be provided for synchronizing the instant at which the scene light is first admitted to the film plane at the commencement of the exposure interval to the transmission of the sonar ranging signal in order to control the strobe fire time at the appropriate aperture. Such a system for firing a flash as a function of a sonar determined ranging signal is also disclosed in U.S. Pat. No. 4,192,587, entitled "Proportional Fill Flash", by Arthur G. LaRocque et al., issued Mar. 11, 1980, in common assignment herewith and now incorporated by reference herein in its entirety. An LED in combination with first light detect apertures in the shutter blade mechanism operate to synchronize the transmission of light from the LED to a photocell in correspondence with the instant that scene light is first admitted by the primary apertures in the shutter blades at the commencement of the exposure interval. Such systems, however, generally have not embodied digital scene light integrating techniques which require extended integration lead times for the integration levels to come within required operating voltage ranges before digital pulse counting can begin. This extended integration lead time may thus introduce an error in the precise detection of the instant that scene light is first admitted to the film plane at the commencement of the exposure interval so as to subsequently cause an error in the time at which the flash is fired thereby causing the flash to be fired at an aperture value which might not be properly correlated to the camera-to-subject range. Other types of scene light measurement such as the pre-exposure measurement of ambient scene light must also be made in a precise manner without the error that might otherwise result from an extended integration lead time.
One satisfactory solution toward this problem as disclosed in U.S. patent application Ser. No. 198,781, entitled "Prebias for Digital Light Integrator", utilizes an arrangement for prebiasing the light integrator to provide an output therefrom within an operative range of voltages immediately prior to the detection of first light at the commencement of the exposure interval in order to eliminate any potential errors which might otherwise result in the detection of first light at the commencement of the exposure interval or in any other scene related measurement resulting from the integration lead time required for the digital type of light integrating arrangement to reach its operative range of voltages. Such a system although generally satisfactory for most applications may still not be accurate enough for some applications wherein it becomes necessary not only to commence the scene light integration from within an operative range of voltages but instead to commence the scene light integration from a precise voltage within the operative range of voltages in order to eliminate the possibility of missing even a single pulse count by the digital scene light integration counter.
Therefore, it is a primary object of this invention to provide a means for presetting a digital scene light integrator at a precise voltage immediately prior to its integration of scene light in order to assure that the subsequent integration of scene light be accurate even to within a single pulse count.
It is a further object of this invention to preset a digital scene light integrator prior to the actual integration of scene light without prebiasing the scene light detector.
Other objects of the invention will be in part obvious and will in part appear hereinafter. The invention accordingly comprises a mechanism and system possessing a construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.